1. Field
This invention pertains to the field of vision diagnostics, and in particular to a method and apparatus for measuring the topography of a cornea of an eye.
2. Description
Ocular aberrations typically produce unwanted results (bad eyesight) and therefore need to be characterized so as to be adequately treatable.
Accordingly, wavefront measurement systems and methods have been developed for measuring ocular aberrations of an eye. On class of such systems typically provide a probe beam to illuminate the eye and measure the wavefront of light refracted from the eye to measure the total aberrations of the eye.
Since typically 60-70% of ocular aberrations result from imperfections in the cornea, such wavefront measurements can be more valuable if the corneal topography of the eye is known. Topographical measurements of a cornea are typically performed by a corneal topographer. A variety of corneal topographers are known in the art, examples of which are disclosed in U.S. Pat. Nos. 5,062,702 and 6,634,752, which are herein incorporated by reference. It would be useful to provide a combined system for measuring total ocular aberrations and the corneal topography of an eye.
One type of corneal topographer employs a “Placido disk” system. A Placido disk system consists of a series of concentric illuminated rings that are reflected off the cornea and viewed with a detector array, such as a charge-coupled device or video camera. Because of its great simplicity, the Placido disk system has been widely used for measuring corneal topography. A key part of this system is the object surface with rings as well as the spatial distribution and the width of these rings on the surface. The location and width of the rings are computed in such a way that the image of the rings reflected off a reference sphere is a uniform distribution of rings, i.e., rings equally spaced and all with the same width. The radius of curvature of the reference sphere is made equal to the mean radius of the cornea (about 7.8 mm). Then the image of the rings reflected off an aberrated cornea will be distorted rings, and from this distortion it is possible to obtain the shape of the cornea.
Many variations on the Placido disk approach for corneal topography measurements have been developed over the years, examples of which are disclosed in U.S. Pat. Nos. 4,993,826 and 6,601,956, and by Yobani Mejia-Barbosa et al., “Object surface for applying a modified Hartmann test to measure corneal topography,” APPLIED OPTICS, Vol. 40, No. 31 (Nov. 1, 2001) (“Mejia-Barbosa”). Mejia-Barbosa is incorporated herein by reference for all purposes as if fully set forth herein.
One problem in many Placido disk type corneal topographers is that the central region of the corneal surface cannot be detected during the measurement because of the need to provide an opening or aperture in the Placido disk for passing the light reflected from the cornea to the detector array. This is especially disadvantageous because the central optical zone of the cornea in particular determines the refractive power of the eye and typically forms the pass-through point of the visual axis. The so-called Stiles-Crawford effect leads to the consequence that the central corneal zone—which is free from any light patterns during the projection of patterns from a Placido-type light source—plays a special role with respect to the peripheral corneal regions of the eye's projection system. As the opening or aperture is increased in size, this problem is exacerbated.
Another problem in Placido disk type corneal topographers is alignment error (i.e., “vertex error”) between the corneal surface vertex and the design corneal vertex plane. More specifically, the instrument expects the cornea to be located at a particular location long the optical axis of the system with respect to the Placido light sources in order to make accurate calculations of the corneal topography. If an actual cornea being measured is “too close” or “too far” from the instrument, then there is a vertex error that will produce inaccurate corneal topography results, unless this vertex error can be determined and factored into the corneal topography calculations.
Yet another problem with Placido disk type corneal topographers is that the data is obtained from analysis of a series of projected rings. That is, a radial position of the detected ring is compared to a reference position and the comparison is used to determine the corneal shape. However, this only provides radial deviations. While these are azimuthally resolved, they do not provide an adequate measure of the “skew” rays, i.e., those rays which would be deflected in an azimuthal direction. This is an inherent limitation for a system using Placido rings. This limitation is especially significant considering that astigmatism, one of the major classes of ocular aberrations, is known to generate significant skew rays.
Therefore, it would be desirable to provide a combined system for measuring aberrations and a corneal topography of an eye that can address one or more of these problems. It would also be desirable to provide a method of measuring aberrations and a corneal topography of an eye. It would further be desirable to provide a corneal topographer that allows the topography of the entire cornea to be characterized. It would still further be desirable to provide a method of determining vertex errors between a corneal topographer and a cornea being measured. It would even further be desirable to provide a corneal topographer that produces a uniform grid of spots on the detector array when an idealized structure (e.g., a “reference cornea”) is measured.
In one aspect of the invention, a system measures a corneal topography of an eye. The system includes a group of first light sources arranged around a central axis, the group being separated from the axis by a radial distance defining an aperture in the group; a plurality of second light sources; a detector array; and an optical system adapted to provide light from the second light sources through the aperture to a cornea of an eye, and to provide images of the first light sources and images of the second light sources from the cornea, through the aperture, to the detector array. The optical system includes an optical element having a focal length, f. The second light sources are disposed to be in an optical path approximately one focal length, f, away from the optical element.
In another aspect of the invention, a method of measuring aberrations and a corneal topography of an eye comprises: illuminating a cornea of an eye with light from a group of first light sources arranged around a central axis, the group being separated from the axis by a radial distance defining an aperture in the group; illuminating the cornea with light from a plurality of second light sources, the light passing through the aperture, the second light sources located at an optical infinity relative to the cornea; providing a probe beam through the aperture to a retina of the eye; providing images of the first light sources and images of the second light sources from the cornea through the aperture to a detector array; providing light from the probe beam scattered by the retina through the aperture to a wavefront sensor; determining the cornea topography from an output of the detector array; and determining aberrations of the eye from an output of the wavefront sensor.
In yet another aspect of the invention, a method of measuring a corneal topography of an eye comprises: illuminating a cornea of an eye with a group of first light sources arranged around a central axis, the group being separated from the axis by a radial distance defining an aperture in the group; projecting collimated light beams from a plurality of second light sources, through the aperture, to the cornea; providing images of the first light sources and images of the second light sources from the cornea through the opening in the principal surface to a detector array; and determining the cornea topography from an output of the detector array.
In still another aspect of the invention, a method is provided for determining a vertex alignment error for a corneal topographer comprising central light sources to sample a central region of the corneal surface, and a Placido-type light source array to sample an outer region of the corneal surface outside the central area. The method comprises: measuring, using the central light sources, a curvature in an outer ring of the central area of the corneal surface, adjacent the outer region of the corneal surface; measuring reflection locations from the cornea of an innermost set of light sources of the Placido-type light source array; using the measured curvature of the outer ring of the central area of the corneal surface and the measured reflection locations from the cornea of the innermost set of light sources of the Placido-type light source array to calculate a vertex alignment error for each of the innermost set of light sources of the Placido-type light source; and determining the vertex alignment error for the corneal topographer from the calculated vertex alignment error for each of the innermost set of light sources of the Placido-type light source.
In a further aspect of the invention, a system for measuring a topography of a reflective surface, comprises: an optical element disposed about an optical axis and comprising an object side, the optical element defining an object space located on the object side a finite distance from the optical element and an image space conjugate the object space; at least one first light sources disposed an optically finite distance from the object space and at least one second light source disposed at an optical infinity with respect to the object space; the optical element configured to provide an image within the image space when a reflective surface is disposed within the object space.
In still a further aspect of the invention, a system for measuring a topography of a reflective surface, comprises: an optical element having a focal length and disposed about an optical axis, the optical element comprising an object side and an image side, the optical element defining an object space located on the object side a finite distance from the optical element and an image space located on the image side that is conjugate the object space; at least one first light source disposed an optically finite distance from the object space, and at least one second light source disposed on the image side, the second light source located along an optical path approximately one focal length away from the optical element; the optical element configured to provide an image within the image space when a reflective surface is disposed within the object space.